I normally do a single mash so I only take one ph reading and adjust the ph with some lactic acid to get within my target range. now I want to do a brew with step mashing where I need to add hot water to get my mash to the next temp step. each time I add water the ph changes and goes out side of what I want it. Must I add more lactic acid after each step to keep my ph within the range I want it to be ?
If you're trying to hit an optimal pH, then yes, you'll have to adjust after each addition.
However, rather than sticking with the same pH for the entire mash, is generally best to let the pH rise as temperature rises, since the key mash enzymes that work well at higher temperatures also have higher pH optima.
For example, Brewkaiser states beta-amylase (fermentable sugar enzyme) has a optimal temperature range of 63°C, and an optimal pH range of 5.3-5.6pH. For alpha-amylase (non-fermentable sugars), the temperature optimum is 72-75°C and pH 5.6-5.9. (All pH values taken at room temperature.) The same is true for beta-glucan, and two of the protein-degrading enzymes - these have lower optimal temperature ranges and also lower pH ranges.
So, when you dilute the mash to raise the temperature, take into consideration the enzymes that you are targeting and their ideal range. The inadvertent pH raise may still be at or close to target, since you're now targeting different enzymes with higher pH optima.