Welcome to the forum. Although you can post in any forum, the USA forum is here in case of local regs or laws
Best using a proper mixing valve, like esbe.
[DLMURL]http://www.esbe.eu/de/en/~/media/ESBE%20PIM_ESBE%20sync%20BR/Documents/Data%20sheets/GB/Guide%20Rotary_GB_99501362_A_LR.ashx[/DLMURL]
Just use a 28mm 3 port and twist white and grey together.
Reply to the thread, titled "28mm 3 Port Valve, But Not Mid Position." which is posted in UK Plumbers Forums on Plumbers Forums.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.