I've fielded a few questions from people interested in Impetus about how our basing system works. Jason and I initially started out with the 'proper' 12cm frontages for 28mm but found that the battlefield was too cluttered and it got in the way of our fun. So we followed Craig's lead and went for 8cm frontages instead.
Here is a quick rundown for how I've done it and some comments on how to make the basing a bit more flexible.
First up the heavy infantry (FP, in impetus terms). This runs the gamut from heavily armoured troops to almost unarmoured but they all have the common characteristic that their formations are dense, with solid lines. The suggested base depth for these is 4-6cm and you want to give the appearance of a solid mass of troops. I use 8 figs per base for these and have a mix of two ranks of four and some other bases (like the one on the right) have a front rank of five, with figs arranged behind that. The base on the left is a General and I have used less figs, to help it stand out some more. I usually do these 5cm deep but I think 4cm deep is ok.
Some more shooters. The (T) Archers are joined by some (S) Skirmishing archers and a unit of (Art) Artillery. The official base size for S Skirmishers is 3-4cm. I went with 4cm but am beginning to lean toward 3cm and might do that for my upcoming Hellenistic army. I have modeled the S Skirmishers with two figs (because they are VBU 2) but have Lanciarii (VBU 3) for my Romans that have three figs. The Lanciarii are much more effective than those archers and deserve a more impressive looking unit. The ART Artillery can be on a 6-8cm base and mine is on a 6cm base, with the weapon and two crew for a light piece.
Next up Cavalry. This is the topic that caused the most debate but here is what I settled on and I think it gives the best visual effect. On the left, there are (CP) Heavy Cavalry with VBU 6. I modeled these as 4 figs and with 7cm of depth to get them all to fit. The official depth is 8cm but with our frontages that gives a square base. However, I might do that if I ever model up some Knights. In th middle we have (CM) Medium Cavalry with VBU 5. I still model these with four figs (but only 6cm depth) because they are still quite a strong unit on the charge and were pretty respectable cavalry in their time. To the right I have (CL) Light Cavalry Equites Illyricani with Javelins. These I have as three figs on a 6cm deep base. If CL is of really poor quality (eg VBU 2 CL scouts) then they could have only two figs.
That just leaves handling large units. These are represented in Impetus by multiple bases, stacked deep. This will usually be two bases but Pike units can have three. With our 8cm frontages, units can have very long bases this way and an overlarge foot-print, especially related to their frontage. A lot of people suggest having 2nd (and 3rd) bases as one rank of figs and I think that is a good idea. To the right you see my Vikings, which are large units of FP. I based these 11cm deep (so more than twice the depth of my usual FP bases) and did so because I needed to sabot base 25mm rounds onto them. I could have had just three rows of figs but I was already stuck with only three figs to each row and didn't like the visual effect of a 9 man FP unit - that is only one more fig than a single base of my Legionaries! If I did have 4 figs to a row, I would have happily made the second base only one row, rather than two.
I hope this helps people trying to figure out their Impetus basing for 8cm. At the end of the day, it is a matter of individual preference but I suggest that at least all the units that can hold their own in a fight need a decent amount of figs on a base and that bases of only two figs should be avoided for anything other than the VBU 2 dregs of our armies.
Thanks for reading.