Exercise: With an empty tank and an empty BC, float at the surface head-up. Take a full breath of air and hold it. Stop swimming. You should float exactly at eye level - that is, the water should bisect your mask. Add or subtract weight to correct.
That should nail your weighting - and show you how well you've already done in getting your weighting down pat. Sounds like you are already pretty close to nailed.
From there, take six pounds off of your belt and put them in the BC's trim pockets on either side of the tank to simulate a backplate's weighting... Then work on trim. The BC should never shift you uncontrollably into any position. You should be able to take a position in the water - usually but not always prone and horizontal - and stay there without having to fin or scull your hands.
For a really advanced trim exercise, try this: Assume a prone (horizontal) position midway in the water column and float there, like a skydiver. If your knees are straight, you should tilt up. If you bring your heels to your butt, you should tilt down. This is aided by heavy fins like Scubapro Jets or ankle weights if you have lightweight plastic fins.
Of course, sculling or finning can screw up the results, so don't do either when running those tests.
The "bend knees to tilt up or down" exercise is called a Center of Gravity Drill, or a CofG drill.
Dive Right In, what does your shop charge for a 7' primary hose? What about a whole hose package: 7' primary, 22" or 24" backup, 24" lp inflator, and 24" hp spg hose? Can you get us a good deal on brass and glass Thermo spgs?