Could you provide a formula to use so that I can see how much I’ll be down the second I’ve placed a bet?

Here’s the bet details.

(Bet at £1.50 Per Point, Direction: Long)

Sell Price 139.65p
Buy Price 142.35p
Additional Spread (for Guaranteed Stop Loss) = 0.994%
Stop Loss 18 Points

* By the way, you’ll see that I used a wide stop-loss for this bet (about 12.5%) that’s not an error. I’m profitably spread betting using wide stops at present, and have been doing so from the start of this year.

The way I calculate this, I think you should be £6.17 down right from the start;
The spread between buy and sell : 142.35 – 139.65 = 2.7
Additional spread for guaranteed stop : 0.994 * 142.35 /100= 1.415
1.415+2.7= 4.115
You are long at £1.5 a point so multiplying by 1.5 gives £6.17
Without the guaranteed stop you would be down £4.05 right from the start (2.7*1.5)