I agree entirely, and most of our games follow that format.
I am basing my forces on the British Waterloo OOB. Within that force a total of 41 Royal Engineers and 800 Sappers & Miners were deployed. Which at 20:1 would be 42 figures for the entire force. I am planning 2 'Companies' of five figures; an engineer and 4 sappers which works out at about 24% of the force. This matches quite well with my other arms; infantry 38%, cavalry 38% and artillery28% of the OOB.
However, within our group there are often spirited points based discusions and I was hoping to head this off.
The compromise we seem to have reached is to cost a company as an artillery model at 30 points, which I suppose if you are giving engineers 'guard' morale factors is not far off 'pricing' them at guards points.