2 ignition pulses from the coil per revolution of the crankshaft. It takes 2 revolutions of the crankshaft to fire all four cylinders.
what software do you use, by the way?
if pickup is on cylinder wire: divide frequency by 2, as it is 1 spark per 2 revolutions
if pickup is on the out-of-one-coli-to-distributor wire -> it will be 2 sparks per revolution for 4-cylinder engine
If your interest is in finding out the rpms speed of the engine, the tach will tell you that directly. Thereās no need to do any calculations.
If your interest is in finding out the ratio between the crankshaft speed and the speed of any of the belt driven components, itās simple arithmetic. Measure the diameter of the crankshaft pulley. Then measure the diameter of the pulley on the component in question.
(a) Divide the smaller of the two into the larger of the two, and youāll get a multiplier to calculate the speed of the smaller one relative to the speed of the larger one.
(b) Divide the larger of the two into the smaller of the two to get a multiplier to calculate the speed of the larger one from the speed of the smaller one.
Then, using the speed as measured by the tachometer, simply do the multiplication.
If the crankshaft is the larger pulley, use (a) to find the component rpm.
If the crankshaft is the smaller of the pulley, use (b).
Caveat: Iām old. With medical issues. Sometimes I get confused. I will not be at all offended if one of you young pups double checks my formulas. Please. 
sox to record, audacity to analyze.
Duh. I wasnāt thinking.
825 is a more believable number, but it doesnāt match what I calculated from reading the photo tachometer at the same time and multiplying by 2.6 or the suspect number I read by trying to read the drive shaft directly.
Listening to the whole thing it definitely speeds up over time. At minute 4 itās nearly 1200.
well, this is as accurate as it gets 
you could go for down-to-fractions measure on your pulley down below and up, then making your better guess on multiplier, but unlikely you would match electronics in accuracy
anyway, 825 seems to be quite a reasonable number for 4-cylinder
what is the target RPM?
I didnāt think about it until this failed test, but the VECI (which I had to purchase from Toyota, the original long gone) sez 700.
presumably, your ā87 Toyota pickupā has injectors, not carburator, so it is some OBD gen-1 system in placeā¦
quite usual problem leading to high idle rpms is in intake leak, which engine compensates by raising RPMs
āsmoke machineā would be one choice to look for leaks, but there are others too: https://www.youtube.com/watch?v=LoZamqAArME
if you have a suspected place, testing with propane or carb cleaner is probably the easies to do
My '89 Toyota pickup had a carburetor. My guess is that the '87 did too.
I checked WiKi and they tell injectors were on some of [many] engines it has
if it is carburator, it goes down to adjusting its idle controls
Wrong. Post must be at least 10 characters.
Theyāre already all the way out. Manuals point to dashpot as a possibility. I think the general opinion here is that there is a leak and I need a re-build - sigh!
hey, carbs are picky, but in the end lesser mystery than OBDs
I used to have carb-based cars of 1967, 1989 years before, and it was more fun than modern ones (and grass was greener too!)
make sure you do not have obvious leaks on your vacuum hoses, as you will end up with erratic idle if you do
adjusting carb, I would start from making sure it is clean and using carb cleaner spray to clean any channels if it needs cleaning first
ideally, you would find the actual adjustment procedure in the repair manual, the rest I will just recall how it applied to my 1989 Opel Kadett (keen to GM in US)
I would hve to find 2 screw: one is setting stop-point for the choke, another one would tune amount of air going AROUND choke at idle
I would have to tune first screw to be tad-bit before it would start opening choke, leave it there as a initial point
after that, tune air until you get your idle slightly under your target
net, go back to the choke position screw and get it in until you zero in on your target RPMs
DISCLAIMER: this is all was done on my 1.3L 4-cylinder Opel, similar process was used on 1967 Moskvich
ah, more on this
I recall one of carbs I worked on (I believe Opel one) had a fixed dosage for air at idle, and adjusting screw was tuning fuel only, while anotehr car had more adjustments and one could work on fuel and air adjustments separately, which was really a pain
I would carefully check how many adjusters you have and what they are likely doing before touching anything
825 -> 700 is not much adjustment you really need
ideally, this will be all done on the screw setting the initial choke position: if you unscrew it letās say quarter turn and see decrease in RPMs, this may be everything you need and you would not risk touching idle mixture adjustments, which are likely more involved