A candle bridge mould is one of the most difficult moulds to use successfully. The problems relate to the kinds of work you are trying to do, the size of the glass to put onto the mould, the shape of the piece and the stability of the resulting piece.
With a candle bridge you are trying to do a combination of slumping and draping at the same time. You are slumping into the middle and draping over the curved sides.
Additionally the candle mould requires the glass to fall into a small opening and this requires long soak times. Long soak times mean the glass that is draping stretches while the central portion is trying to fall into the opening. Of course, if you don't want the depression to be flat, you don't have to soak so long and the stretching effects on the draping part of the of the glass won't be so great.
It would seem logical to measure the mould around the drapery curve (or arc of the mould) and to the shape of the ends, but experience has shown me that this leads to glass that is too long along the sides and bent at the ends. So I cut my blanks for candle moulds as a rectangle without curved ends and then round the corners of the rectangle just a little by nipping them with my grozing pliers before fusing.
If you measure along the top and along the length of the mould you have a piece of glass that will be increasing in length at the draping part of the mould, meaning that it will fall off the curve and onto the draft (or side) of the mould. The draft is an angle from the vertical. Good moulds are made with a draft so that if glass were to fall over the edge it still will be possible to get the glass off the mould.
The draft on a mould means the diameter of a circular one is greater at the base than it is at the rim. And it is common to measure only the diameter at the rim. In the same way the dimensions at the outside base of a rectangular mould are larger than the rim of the mould.
Back to the rectangular candle mould. The draft on this means that measuring the base of the mould is slightly wider than the curved part of the mould, but less than if measured around its curved portion.
Experience has shown that in the case of the candle moulds measuring the width of the mould is sufficient. There is enough height in the moulds I have used that it does not make any functional difference if the glass does not reach the bottom of the curve on the mould. It is better than hanging off the edge.
The length of the glass should be no longer than the shortest part of the mould's length. Cutting a curve into the glass to allow a small overhang produces a depressed lip because of the length of the soak required for the slump into the small aperture of the candle depressions.
My soak for candle moulds is 90 minutes at my process temperature. This gives me a flat depressed area for the candle to sit, but it also means that the draping glass has been stretching. And it also means that the glass will drape unevenly as the various colours absorb heat differently allowing some parts of the glass to stretch more than others.
The placing of the glass on the mould is absolutely critical. It must be exactly parallel to the sides of the mould. Any slight movement from that will induce a twist in the resulting piece allowing it to rock. Arranging it exactly right and placing some kiln washed furniture at each side to keep it in place until it begins to slump is an important aid.
The glass will begin to bend before it sticks to the kiln furniture.
I have never been able to get a stable candle mould whether from 3mm or 6mm thick glass. I always have to grind the base a little to make a stable piece. I take it as part of the process, but careful placing reduces the work.