How to calculate an amp's power usage?

If the amp is 1 ohm stable, then 90 amps should handle peak at that rating.  RMS should be 1/2 of that, or 45 amps at 1 ohm.  If you have a 2 ohm load attached to the amp, the peak should be more like 45 amps peak and 22.5 amps RMS.  That is using a set of assumptions which may not be accurate in real life.

But then there are these possibilities:  that at 1 ohm peak is higher than 90 amps but those fuses were used for basically two reasons.  One, the convenience factor of using three easily attainable fuses, and two, that the physical limits of the amplifier are reached at that current level.  I don't know exactly what is the case here.  There also is the limitations of the car's power supply, which may not be able to provide such a high current in actual operation.  And it could be that the 2 ohm load is not actually half of the 1 ohm rating for this amp, but maybe 2/3 or so.  (The Diamond site isn't working for me to look up specs).

But my bet is that you will end up fine with your current wiring setup.  You only have to blow the fuse once to know that an upgrade may be needed.

Well, it's got a regulated power supply, so the amp draw should go up as voltage drops, so that it can output the same wattage...

Now, my 4ch amp is going to run at 4 ohms and I believe it's 2ohm stable, and it's fused for 40a.  It's a class AB amp, but I'm guessing it'll probably draw closer to 20a.  It's only going to be putting out 50x4.

So I guess I wasn't too far off in what I was thinking with the formulas.  I know there are a lot of other factors that are nearly impossible to calculate and/or figure into the equation, but it's one of those general "rule of thumb" things that you can kind of get a general assumption out of.  With a 90a and a 40a amp, hopefully my real draw will be more like 60a with the two amps together.  I guess if I start blowing 80a fuses with my current distro block, I might have to upgrade to one with mini-ANL fuses in it so it can handle more amperage.

have a look at ohms law and you should be able to figure it out with the specs u have.

https://www.the12volt.com/ohm/ohmslaw.asp

Remember too that music is dynamic so you will very rarely, if ever be drawing that much current from your amps. Your 20a calculation is probably a lot less during normal listening conditions.