Peppers,
That extra three feet should not make any difference at all. If she is OK at 40 ft, she will do fine at 43 feet. If she is barely making it to the plate at 40 ft and you think she wont at 43 ft, then she really needs to work on building more speed.
I had students that only pitched during their rec league season. They would start up their lessons a month or so before the season started and stop at the end of the season. The younger ones were only allowed to throw 3 innings a day.
What I had most of my students do was to mark the number, keep some kind of record, of how many pitches they threw in practice at home every day.
What I told them to do was simple; increase the number of pitches a day by 10 each week. Week 1- 40 pitches, week 2- 60 and so on. Keep increasing that numberuntil you get to the point you are pitching 150% of how ever many pitches you might throw if you are having a 'Bad' game. That way, even the worst game and that many pitches would not be even as stressful as a normal days practice session.
You have to build up your 'Pitching stamina'. You dont start out running a 20 mile marathon, you have to work out and build yourself up tothe point you can do that. Same goes for how many pitches you can safely throw in a day, a game or a weekend.
I have seen 14u pitchers that started the game throwing BB's noone could touch and then, after two innings of that, they could not throw a pitch that didnt hit the dirt 3 feet in front of home plate. Why? Because they lost their grip strength because they never practiced, at least nowhere near enough or the only practice they got was in their session with the instructor.
I have also had a few students that practiced at home every day and threw 250+ pitches each time. Those pitchers could throw hard for 5 games a day and not get injured, tired yes but not hurt and still had their speed and grip strength at the end of the day.
How much a pitcher can throw safely at any age depends on their work ethics AND ONLY THEIR WORK ETHICS. It has ZERO to do with their DNA, their genetics or anything of that sort. IT'S ALL ABOUT WORK ETHICS.
Here is a favorite saying I came up with many years ago. I hope you find it useful.
"PITCHING IS LIKE OWNING A PIGGY BANK;
ALL YOU CAN EVER EXPECT TO GET OUT OF IT IS
EXACTLY AS MUCH AS YOU HAVE BEEN WILLING TO PUT INTO IT" WINNING FAST PITCH SOFTBALL