For approach shots I consider the flagstick to be 7 feet high and move the triangle appropriately. For other shots I just wing it by experience on the inset map.
To know if your numbers are close, you have to make a lot of shots so as to average out the deviations. I shot a LOT of CTTHs and kept numbers.
Calculating closely sounds picky, but you really need it if you want to be within 10 feet.