Dear Anwar,   it's almost ...

Dear Anwar,

it's almost impractical to standardize this issue. The purpose of adding dosing acid to the feed is to neutralize the available alkalis in the feed water and reduce scale formation by carbonates.. there is no standard mathematics and it depends on your feed water analysis and the design of your plant. 

I suggest you start with low acid concentrations and low dose on a side stream for a try before you put it in service and start your dosing. 

Don't forget that acid for pretreatment to lower PH in feed while caustic for product PH readjustment in product line. You can use the membrane manufacturer program to assist in the initial estimation after you input your feed water analysis. 

I. E. The more acid you will use in the feed, will require more caustic in the product to adjust PH later in the product. If you don't use acid in feed, you may not require caustic to readjust PH. 

Tip: if you still follow me, the two amounts are almost the same after you count the mass balance, concentration of chemicals, acid valency/base and your recovery ratio (not all the acid you dose in feed will go to product, depending on your recovery) 

N. B. The variation in PH after adding the acid is going to be high in case present alkalis are low and the TDS is low.  That's in the feed line, same for caustic in the product line. 

Good luck