The amount is going to depend on how low you need to go. IIRC, the general rule of thumb is 1% per 0.1 drop in pH. So, if you needed to adjust pH by 0.5, you would need to include 5% of your grist as acid malt. That being said, acid malt is pilsner malt with lactic acid, so if you need to acidify a lot, you could potentially get the same flavor contribution as using lactic acid. For my wheat beer recipe, I used to use about 1mL per gallon and that was right at the threshold where I could smell it fleetingly in the finished product. So, I switched to phosphoric and it's a non-issue.