Heh. Well there are reasons people are told DON'T FUCK WITH THE PROGRAM but they don't apply if you're dealing w/ someone who knows what they're doing. But in your case, yeah you could use leg press as your "squat" movement for one day. As long as you don't get too far into isolation type stuff, most changes are fine. Just train progressively and apply the basic framework to new exercises. Note thought that leg press may/may not help your back squat poundage. And that drives the program. So you wind up w/ Monday (back squat), Wednesday (front squat or light back squat), Friday (leg press). Hopefully your back squat will just go up week to week on its own. Because normally, you'd have a volume squat day (5x5) to drive the intensity squat day (1x5). EVentually, you're into the advanced versions and doing 5x5 sets across on Monday AND Friday b/c you need the extra volume to push your top set up. So lets say you're at this point. But you're doing a leg press day. So you're not really using something to push your back squat. You could come in Monday and do 5x5 back squat, then do the rest of your stuff through the week. Next Monday, maybe try for a new 1x5 PR. Then the next Monday, try a new 5x5 PR. Dunno. Just an idea. Or you could just keep doing 5 sets of 5 every Monday and see how it goes. Or try 5x5 on Monday, then next Monday, 6x5 w/ the same weight, then 7x5 w/ the same weight the next week, and then add 10 pounds and go back down to 5x5 or something. Lots of ideas really once you get the big picture.