Like many disciplines, math in aviation is just a means to an end. Indeed, in nearly all cases, you will use the result of a calculation to make a decision: Do I have enough fuel, flying as I am now? (really important one!), Do I have enough runway length for these conditions? Am I within weight and balance limits? Destination circuit height is 2500 feet, and I'm at 4500 feet, ten miles back, flying at 120 knots, will 300 FPM get me down in time? To identify just a few...
When you have determined what the question is, and how to get the answer, you may apply math to the process. How much math? How precise does the answer need to be? A lot of the decisions you will need to make are better made more quickly (within the minute), then confirmed with precise information at the first appropriate opportunity. If it's a weight and balance, then yes, your first appropriate opportunity is before takeoff! However, if you suddenly suspect that a diversion might be necessary, spending ten minutes making that decision is probably going to really worsen an already bad situation. You gotta calculate your fuel, and decide if the new airport's runway is long enough with little delay, so you can make your decision.
Then, once you have made your decision, and the plane is flying as needed, you might wish to confirm your quick math (guess), to confirm your decision.
So get good at guessing a reasonable answer, then using math to confirm it. If you have 17 gallons aboard, and are burning 8 GPH, and your destination is 180 miles and you're groundspeed is 90 MHP away yet, are your going to overfly an enroute fuel stop? I sure would not! Your quick math says you will have enough fuel, though not the required reserves. But that quick math tells me that it's a poor decision to pass on a fuel stop, as your margin will be very tight when you get there.You just made a decision, based upon math, but the math was really simple, and you did not write anything down.
Devote more effort into understanding why the decision must be made, and what's at stake if you get it wrong. What are the variables? Which things have changed since your trip plan, and takeoff? Guess your answer, make your decision, then confirm it by double checking with "better" math. The key is to always be "ahead" of the plane and the flight enough, that your decisions can be made, acted upon, and then confirmed, rather than a rushed guess, which under stress was later found to be wrong, and the whole decision has to be made again, with much less margin now...
My VFR experience has always been that with a good plan for the flight before you leave the ground, you are rarely doing much math once underway....