Did you use his calculator (
http://http://tools.sharpcap.co.uk/)? According to my calc there, @ F5/100 gain under bortle 4.2 skies your exposure lengths could be shorter, depending on the acceptable noise percentage (5% or 2%).
(Formula = %acceptable noise multiplier x read noise squared / sky electron rate).
Lum: 22 to 54 sec
RGB: 65 to 162 sec
NB: 2000 to 5000 sec (30 minutes plus, getting past the limits of the camera exposure times)
This is based on calculations by Dr Glover, using Bortle 4.2 skies, F5 scope, 100 gain giving 2.0 read noise, 3.8 pixel size, 3nm NB & LRGB filters.
Increasing the gain up to 150 brings the read noise down to ~1.6, reducing the NB time to about 20 minutes. While these may be 'optimum' times, you do have to consider your mount capabilities and other factors that may affect guiding or the quality of the final results - it's no good using these figures if you end up throwing out half your data. Personally I would stick to 10 min subs and just take more subs.
Just note that the calculations are greatly affected by the bortle level used, I underestimate mine for the calculations on my spreadsheet. My skies are supposed to be ~4.3 but I use bortle 5 to allow for ever increasing light pollution. Using Bortle 5 for you brings the 'optimal' times for 3nm NB down to ~10 minutes.
Cheers,
Mark