I've actually spent the afternoon revising the formula to account for fire arc. Also revamped are speed, turrets, and WB vs Lance price balance, so it might actually have a chance at that ship! Will work it out when I get home. Edit: 455pts not accounting for 90'. Call it 480pts.
I've put in 21 IN ships so far, and it's worked out surprisingly well! Apart from the Mars and Dominator being under by 11-15pts (and that is purely down to the rulebook premium on NCs compared to torps - removing the premium would put them back within spec), the only 2 ships so far have come out as being more than 5pts wrong by my revised formula are the Emperor (only 7.5pts out) and the Vanquisher - and actually, I find I have to agree with it. 300pts is too few for what you get. With 20cm speed it should be around 320-325pts. With 15cm speed, 310.
If anyone would like to have a look at my revised formula, I'll be happy to share it. At the moment it's just a spreadsheet, and I've only tested on IN ships so far.