Thanks all. My 28's are based on 50mm bases with two ranks of three. Three such bases give me the 18 figure regiment. (I use 50mm bases in preference to the 45mm bases supplied by Perry).
For my 12mm, which I bought to use with hexes, I use the Kallistra system of 8 figures per bases in two ranks on 40mm frontages . Two of these bases side-by-side give me my regiment (i.e. 80mm frontage).
If I do something biggere such as Black Powder, I use 3 bases to give a frontage of 120mm.
In practical terms my 16 figure 12mm units and 18 figure 28mm units have no impact in game effect. Halving the unit frontage from 150mm to 80mm does help with the game conversion of inches to centimetres.