If you've managed to work out how to fill in the SRG 1104 at all you're doing better than me - I've been trying to work it out for months now.
I have never suceeded in working out the correct amount to pay the CAA. I just give them my credit card number and leave the amount blank on the forms, on the grounds that (a) that works and (b) that's better than have them sit on the paper work for several months and then send it back because I'd calculated the amount wrongly.