QTR-Quadtone RIP

I am wondering if you guys cab help me. You seem most knowleagable group out there. I have made flat bad printer using epson 2200.I would like to print on dark garments. In order to do that I need to print white underbase and than color image on top. QTR would be perfect for that but unfortunatelly I can't use QTR since it can't print color image. I have tried QTR for underbase and gutenprint for color image and it worked but I had problem with registration. So I tried using gutenprint for both and I got it almost good enough. Registration is perfect but there is no enough white ink. I have made gradient swatch from magenta to white and two more using cyan and black. I put white ink ink in lm, lc and lb channels. Than I printed those swatches and white was printed on one half of the swatch. Than I took rgb value for spot where white was most intense. Next part was to apply those values on image I wanted to print. I use photoshop to to edit image. First I convert to grayscale than duotone and than tritone. Here I got 3 swatches and for each I change rgb values that I mentioned above. Finally I change mode to rgb and send to gutenprint for print. It prints white but not enough. Does anybody knows how I can adjust gutenprint to put down more ink or some other idea. Somebody mentioned that ghostscript is good for this. I have downloaded it but I can't see any settings for individual channels. Can you help with some ideas
Regards
Bo