I used to do carpentry for a living, Started in a cabinet shop, then insurance claims, then new home const, then reno's and such.
About 2 years ago I quit and took an equipment operating job, now I drive mine truck.
Helping a friend build a fence and I could not remember to to evenly space pickets on the gate!
Well I remembered but it took a while after doing it wrong twice!
Using 1x6, put your fist one on, then measure the distance left (86" for us, 91.5 total) then add your space desired (1.5) between boards to the width of a board (5.5 + 1.5= 7) divide the distace by the deisired spcae (86/7=12.2857) gives number of boards needed, round off to higher (closer space then desired) or lower (wider space then desired).
We went higher so 12, length divide boards (86/12= 7.1666666667) so every 7.166666667 inches there is a board.
Really not that hard, but the ol brain just weren't percolating.
"Use it or lose it!" as they say. I guess we would go nuts if all the formulas for things we once used for different things were allowed to rattle around front of mind. You didn't forget the gate needs hinges right?
The irrigation questions on this site frustrate me at times. I used to run all the hydraulic, pressure loss and flow calculations in my head. Now I really have to look most of them up to be sure.