How much a fat shot affects a shot off of a mat depends on your angle of attack. My angle of attack is so shallow that I can get away with ridiculously fat shots on a mat (or any hard surface) and barely even know that I am hitting it fat.
Once I get on grass (especially on wet ground) that same impact is fat enough to be a disaster. When I practice off of my mat at home I always also hit some balls off of the ground just to make sure I'm not grooving impacts that are not going to work on the course.
Yeah, I know what you mean, that's the danger of practicing on mats. That is why I always lay a strip of masking tape behind my ball so I can monitor my swing bottom.
I'm thinking that 3/4" while FAR from ideal shouldn't be too much of a disaster on the occasions that it happens, I mean it's not like I'm hitting 4" behind the ball and laying sod over it.