Sorry-I can't help myself. I always do a rough mental calculation to assure myself that the box of silicon chips probably gave me the correct answer. It's too easy to hit for example the x button when you really meant to divide. So if I'm adding 256 L of Avgas and I need to calculate the added weight I mentally note it should be less than 256kg but more than half of 256kg. I then multiply 256 by 0.7 and arrive at 179.2 so it's probably correct. If I arrive at 365.7 I know I've made a mistake,in this case divided instead of multiplying.
Two more handy rule of thumbs.
For 75% power use the rule of 48. MP in Inches plus RPM hundreds will be pretty close to 75% power if they total 48 eg 24 inches MP plus 24 hundred revs is about 75% power so is 23plus 25 or 21 plus 27
For TAS use 2% of the IAS X altitude in thousands and then add this result to the IAS
eg Indicating 135 knots @ 9500 feet you get 2% of 135 = 2.7 X 9.5 = 25.65 plus 135 =160.65 knots. This doesn't allow for temperature effects but it's pretty close to right.
Cheers RA