Just to be clear, P-990 meant shooting well off your hands while standing, and not literally being off your feet (the sitting and prone positions).
Rehearse your positions at your local range. Develop a sequence of moves into each position that you memorize like a martial arts kata, and then go through every one, without fail, every time. And not just the obvious stuff, like setting up your ammo and sling and putting on your "ears" and making your sight come-ups and windage adjustment, but include taking out and positioning your stop watch, score book, and even your pencil. Put a target number under your target if your club doesn't have them. This is so you stay in the habit of checking that number as you bring the gun into position. Having a routine like this for each position will stop you from leaving something out and can save you from cross-firing if you get excited during a match. Do the same thing dry firing in the basement at home (10 foot practice target for standing and sitting attached).
Don't ever make any sort of last minute changes in equipment or routine just before an actual match. That's a rookie mistake. (Don't ask me how I know.) If, at the last second, you suddenly get this great idea for something that might raise your score, write it down and try it at your next rehearsal, but never do it for the first time at a match.
Check your zeros and come-ups at the local range. My prone windage zero is always about an moa right of my sitting and offhand due to some fluke in my shoulder geometry that introduces a recoil moment that moves the POI left in prone. With iron sights, at 600 yards I am always a couple of moa below table or computer predicted come-ups just because of how my eyes find the bottom edge of the bull. Use a computer to say how high your bullet should be on the 100 yard paper when you are zeroed at 600 yards. It will be around 18". Just make your backer tall enough to capture that on a target center placed at the predicted height above the aiming center so you can score it and make any necessary adjustment.