Um... I'm not sure I'm tracking your statement, but I'll give it a run...
1MOA = ~1" / 100yrds. So If you're looking for a ratio, you can consider it this way: (Real range) * 1"/100yrds = inches/ 1MOA at that range. Since you know your drop, you can divide by that expression to convert your inches of drop to MOA --> 7" / (300yrds * 1" / 100yrds) --> Simplifying that, you can simply "drop the zeros in your range" to get a much simpler expression 7 / 3 = 2.333 MOA, which is 9 clicks in a 1/4 MOA per click or 1/4 IPHY per click optic.
That's not a rule of thumb, that's just a combination of a simple unit conversion and a simple ratio expression... The only "rule of thumb" involved there is the use of ~1" at 100yrds = 1MOA, when in reality, 1MOA dispersion on a normal target face at 100yrds is 1.047197559...". For most shooters, that ~4.7% deviation doesn't matter much, so it's neglected - much like how you were able to neglect your difference between 285yrds and 300yrds, and between 2.333MOA and 2.25MOA... One of those aspects slid you one direction, the other slid you back the other way, and the impact was on target.