0.5oz / 14ml of 88% sounds like far too much to me - it will really make the beer sour. I recently added 2ml of lactic acid to a batch of witbier and the beer was pleasantly sour. I could probably have doubled it to 4ml but then it would have been assertively sour. More than that I think the sourness would be overwhelming.
But it's a personal preference - you should try for yourself to find out how much sourness you like.
If you have a graduated pipette or a syringe or some other way to measure small quantities of liquid, you can take a bottle of similar beer and add lactic acid in small increments until you find the sourness level that you like. You may also find it helpful to dilute the acid first to avoid adding too much, e.g. mix 1ml acid in 9ml of water, to give 10x dilution. Each milliliter of solution you add is then only 0.1ml acid. Once you find the sourness you like, scale the quantity up to 5 gallons.
For example, if you find the sourness to be right after adding 2ml of the solution into 16oz of beer, then to scale that up you would use
= 40 (16oz bottles in a batch) * 2ml / 10 (dilution factor)
= 8ml 88% lactic acid required for the whole batch
Other sources claim higher amounts:
88% Lactic Acid - Lactic acid makes up between .18%-.52% in Gueuze
(according to Wild Brews). This would work out to about 2.4 oz of this
lactic acid in 5 gallons to get .3% lactic acid. A few drops in a
small sample gave a classic sour twang, but there was also a hint of
butteriness. I wonder if they use pediococcus in the production and
some diacetyl makes it in. If it is the diacetyl , then adding this
acid early in the secondary fermentation to give the Brett time to
deal with it would be a good idea. When I added enough to make the
beer really sour there was a lingering aftertaste, but it was hard to
put my finger on exactly what it was.
- Brewing sour beer with acid
So, 0.56oz may not be out of the question, but I would still try yourself to see.
Adding the acid during fermentation rather than at bottling time gives a smoother tang.