During toilet re-fill, my bowl fills faster than the tank and sometimes the bowl overflows because I have a slow-draining sewer (partially clogged septic field lines). The toilet keeps running and the float valve doesn't reach shut-off level. The re-fill line is plumbed 100% into the overfill tube. Should I divert some of the re-fill flow directly into the tank? Septic field lines are pricey and I'm trying to limp along. Thanks!!!
Are you sure that 100% of the fill water goes to the overfill tube? Look closely, most fill valves fill the tank through ports at the base of the fill valve which are not readily apparent to casual observation.
If your toilet keeps running and the "float valve doesn't reach shut-off level" don't blame the float valve (aka ballcock valve), it's doing what it is supposed to do. You should be looking at the flush valve.
Anyhoo, you need to find and fix the root cause and I suspect something other than your septic lines. A properly functioning toilet cannot overflow because the fill valve directs too much water into the bowl, it is simple physics. I suspect something else, such as a partially plugged toilet trap. It could be the septic lines, but the whole house would be backed up and you did not say that was the case.
If there is a float on the chain, raise it or remove it.
You could detach the fill tube and let it just discharge into the holding tank. But:
Be aware that if that line sits below the water level, you're likely violating code (there must be an air gap between the fill level and this tube, so that it can never let standing water siphon back into the water supply; like kitchen faucets).
Try one of these at a time and see where it gets you.
Another option is adding a tube clamp, to retard the flow of the fill tube. Good toilet repair kits come with one already attached. You need one large enough that it will still allow some water to pass through it when it's set at the first indent (there's 'teeth' on these that make them adjustable). The one pictured at the bottom looks good; can't go wrong with too big.
Available online or, if you're lucky, your local store might carry the kind specifically made for this (the part that comes with the kits).