This is just my opinion, so take it with a pinch of salt.
for me, its not ALL about "muscle memory" , its about Perceived Sensitivity: How the game world feels in relation to your physical movements.
for example, I will train in kovaaks on 110FOV 4:3 , and play most games on as close to 90FOV 4:3 as possible.
Even though I just trained for multiple hours on 110FOV 4:3 , I can move to a game of titan fall, CS, COD, BF... and it will feel just as comfortable, if you have built up a good foundation of muscle memory and perceived sensitivity.
With good and efficient practice, you dont think about what is happening with your mouse, hand or mouse pad, you focus on what is going on in the game, and how it FEELS.
In aim training games, practising both high and Low FOVs are a good idea(I usually practice on what is comfortable) , and then play your game on your desired FOV, and scale the sensitivity by monitor distance Vertical 0%
.
With games that have very low FOVS, like Horizontal 80 degrees or less, like fortnite, I would not scale all sensitivities in the game by monitor distance, I would scale the hipfire sensitivity by CM/360, and the rest by monitor distance. if I was to scale fortnite by monitor distance, i'd have a nearly 60cm/360 degree turn, and that is just not very feasible.