The optimal range is probably between about 3.6 and 4.0, however, the yeast can function well over a very wide range of pH. What this means is that you don't have to worry much about high pH at the start of fermentation. When I mix up a must for a traditional mead, the pH is usually fairly high (4.5 - 5.5). Yeast can function just fine at that level, and after the yeast are pitched, they will secrete organic acids and produce carbonic acid (CO2) and will rapidly lower the pH to a range that discourages competitive organisms. Usually the pH will drop dramatically within 24-48 hours, and in fact the pH can sometimes drop too low.
When the pH goes below about 3.2 it starts to stress the yeast, and by the time the pH is below 3.0 it can actually stall the fermentation. A pH that is too low, is much more problematic than one that is a little high. This is why we discourage the addition of acid blend at the beginning of fermentation. The acid will lower the pH and then when the yeast get to work, they will secrete more acids, lowering the pH even more, which can sometimes cause the fermentation to stick.
So my routine is to add no acids at the beginning and to monitor the pH during the first 48 hours. If the pH drops below about 3.2, I'll add potassium bicarbonate the bring it back up to around 3.3 or 3.4 to avoid stressed yeast.
After fermentation is complete, ideally the pH should be below 4.0 as this will help protect against spoilage organisms. However, I wouldn't add acid to achieve a number; I'd add it by taste if needed to brighten the flavor.
I hope that helps.