Recommandation : Aprox 30 gr on 40 x 50 size of surface.
The main point is you want to lay down as much white ink as possible on the first pass for the underbase on polyester. You will need to apply more ink than on a 100% cotton shirt. How much white ink you can lay down in a single pass will be dependent up on your printer and the your RIP. Some printers, like the Epson default settings will not allow as much ink should have on. You will have to do your testing with the type of printer you have and your type of polyester tshirt.
Working on conjunction with the amount of ink the printer can physically lay down is the RIP. The RIP will also limit how much ink you can put on. Polyester will require 1.5-3 times the white ink to achieve a great looking finished product when compared to a 100% cotton shirt. Testing with your own equipment and settings will be required. Laying down too much white ink can result in the white ink not being kicked over” enough and then when the CMYK is applied it will look great on the printer platen, but when you heat press the ink it will blend into the white causing a dulling and image clarity degradation. So, finding the optimal amount of white ink for your printer will take some testing on your printer and RIP.