I'm trying several pizza crust recipes at an altitude of 4,500' (about 1500 meters). All of the recipes I have are presumably for sea level, and the results are as expected - too much yeast for this altitude causes the dough to over-rise and then collapse. Can anyone suggest how much to reduce the yeast at this altitude - 30%, 40%, 50%, more?
I don't think that reducing the yeast is going to help much. Remember that the yeast is going to reproduce in your dough anyway.
Try rising in a cooler location, even the refrigerator. It'll make your bread better anyway. Start with ice-cold water when you prepare the dough, and then keep it cold overnight. After that, bring it out to room temperature in the morning and let it work from there. Keep an eye on it — don't rely on the clock to tell you when to punch it down for a second rise.
Sugar fermentation occurs more quickly at high altitude so you need to cut back on the time that you're allowing the dough to rise.
Salt controls the action of the yeast so you might be able to increase it slightly since you're talking about pizza crust and not a sweet yeast dough.
Are you using all-purpose flour? If so, try bread flour. The higher gluten content of bread flour typically requires a longer amount of time for rising due to the extra gluten that needs to stretch and expand. Additionally, it will be a stronger dough even when it is expanded.