20 minutes into the mash my first pH reading was 5.7. I was able to get it down to 5.4 using 3/4tsp of Lactic Acid. But that adjustment took place 1/2 way into the mash. Should I not worry about the 30 minutes of the mash sitting at the higher pH?
I can virtually guarantee that you were chasing your tail. The fact is that mash pH varies during a mash. I've noted a fairly consistent tendency for a high initial pH to fall a bit during the mash and a low pH to rise a bit. There seems to be a buffering action that tends to bring mash pH closer to 5.4. So that 5.7 would have naturally fallen if you had given it time.
My strong recommendation is to NOT adjust mash pH during the mash. Make the predetermined adjustments using a program like Bru'n Water and let the pH go. Do monitor the actual room temperature pH during the mash and make note of the final pH at the end of the mash. If its more than a couple of tenths off, make a note of it and add either a dose of acid or alkali to the kettle to bring the pH closer to expectation. But don't chase pH during the mash since it will vary by itself.
Jim, did you measure the kettle wort pH before the boil? I'm curious if the pH was at your target or below.