Instead of cutting the glass round try a square piece instead. Fuse identical pieces from two contrasting colours. The steep angle of this mould will reveal the bottom colour around the edges.
The steep angle of the mould may result in an uneven slumping. Reposition the slumped piece and fire a second time. Use a very low slumping temperature with a long soak
Recommended Firing Chart
Please Read: This firing recommendation has been calculated for use with Bullseye Glass of average viscosity with an even thickness of 4 - 6 mm. The chart shown below is also based on the assumption that you are using a top fired kiln, e.g. Paragon GL22, GL24, BVD Flat bed kilns etc. If you are using a side firing kiln you must allow more time in the first ramp.