With a FBL gyro you really shouldn't use subtrim, or any trim at all. First I would make sure that you have it in heading hold (HH) or headlock mode, whatever you wanna call it, and see if the tail will hold. If not, then adjust your linkage rod, either longer or shorter, whatever way you need to so the tail will hold when you have the heli up to hover speed. You shouldn't use any kind of trim at all.
To get into HH mode, your tail gain settings in your TX need to be above 50. If your settings go from 0-100, then above 50 is HH mode and below 50 is Rate mode. If your settings go from -100 to +100 (like mine do) then anything from 0 to +100 is HH mode and from 0 to -100 is Rate mode. I would make sure you are in HH mode first.
Hope I didn't confuse you. But, like I said, try HH mode first. If it still wont hold then adjust your tail linkage rod until it does hold. Don't use any kind of trim