I have moved on a bit with this frame - at last, and have a question regarding the seat tube sleeve. Below is a mock up of the final layout, the seat tube will be cut off at the top of the sleeve.

I do not have any way of practicing brazing on the sleeve so I have to get it right first time. The sleeve is quite long (115mm top to point) I am worried about getting full penetration. So I am thinking as follows:

1 Drill as big a hole as possible in the sleeve under the top tube junction - maybe 20mm
2 Feed brass into this hole and work downwards until braze is seen at the bottom. Working down to minimise heat on the thin 0.6mm seat tube
3 Invert and do the top section


Does this sound practical? Would it be better to use brass or silver? - I will be filleting the top tube with brass
Am I worrying unnecessarily - might it be easier than I think?
Would it be better to shorten the top end of the sleeve to just beyond the tube junction and then use a seat-tube collar/clamp at the top and leave the bit in between 0.6mm?
Any other suggestions?

Thanks

Mike