You really can't balance the volume of a humbucker whose coils are in series and from that in coil split.
Take note, when in series (full humbucker), you have two coils working. When in coil split, you are grounding one of the coils so that only one of them works. This will also result to a volume loss of around half.
You can try parallel wiring, but you'd need to use two push/pull pots. Two coils will still work and are hum cancelling but they will work in parallel. Volume drop is lesser compared to coil split. Around 30% volume drop lang ito.
There's also another factor being considered when doing coil splits. Take note of the DC resistance of the pickup. Since it will literally be split in half, the overall output and sound will also be affected. You regular strat pickups are around 6-7 kohms DC resistance. If you have a humbucker with a DC resistance of around 8-10 kohms and you decide to do a coil split, you'll result to a single coil with around 4-5 kohms DC resistance. Too low and it will sound thinner and really weak.
Coil splitting can only get you so close to a strat sound. Other variables also come into play like the hardware material of the guitar, scale length, wood, etc..