My first game of Impetus was using 25mm figures and U equals 1 inch. My second game was using 1/72nd scale figures and U equals 1 inch. My third game was using 15mm figures (my preferred scale, or rather the scale of the overwhelming majority of my ancient figures and those of the people I regularly game with) and U equals a piddly 1 centimetre.
In the handling and positioning of troops on the battlefield it is easy to “bump” them by a cm and hard to tell this has happened. It also means that moves seem to be less definitive (think bold, meaningful, determined or just clear).
When I looked at the ratio of frontage of the unit to the distance it could move I became a little concerned. At this point I should state that I have a previously played a lot of DBA and was always snarky about the variation in a 25mm game of DBA versus a 15mm game (there were moves you could do in 25mm that couldn’t be done in 15mm, closing the gate being the main one).
On top of that I was just finding U equal to 1cm was annoyingly fiddly.
What to do?
Unit Frontage to U ratio
For 25mm, with a frontage of 120mm per unit, U equals 1 inch or 25.4mm, ratio 1:0.212.
For 15mm, with a frontage of 80mm per unit, U equals 1cm or 10mm, ratio 1:0.125.
Applying the same ratio as for 25mm scale to the 15mm scale gives a value of U equals 17mm.
The Battlefield (1.2.1)
A standard sized table is 6 foot by 4 foot (180x120cm), well at least for FoG it is. The Advanced Impetus Tournament Rules seem to favour this sized table for 15mm Tournaments (although 300 and 350 point games can use 120x90cm or 120x120cm) and for 300 point 25mm Tournaments.
Battlefield Size 300 Points
Advised minimum battlefield size for 25mm, 300 points is 180x120cm or 71x47U
Advised minimum battlefield size for 15mm, 300 points is 120x90cm or 120x90U
Battlefield Size 500 Points
Advised minimum battlefield size for 25mm, 500 points is 240x180cm or 94x71U
Advised minimum battlefield size for 15mm, 500 points is 180x120cm or 180x120U
Battlefield Size U Equals 17mm
For 300 points minimum battlefield size of 120x90cm is 71x53U
For 500 points the minimum battlefield size of 180x120cm is 106x71U
The Impetus rules give this in terms of U with each command being deployed at least 30U from the centre line and at least 12U from the side edge except for 300 point 25mm games when the deployment from the centre line is reduced to 12U.
Tournament Games Deployment
This is changed for tournament games where armies can be deployed at least 20U from the centre line for a 6x4 table and on a smaller table 15U from the centre line or 25U from the baseline.
What would that mean for a 15mm game played with U equals 17mm on a 6x4 table? The depth of the table in U is 71U. With a 20U deployment from the centreline that would leave just over 15U for the depth of each armies’ deployment zone.
The deepest unit is a triple pike block at 9cm (or just over 5U) so that should present no problems with U equals 17mm on a 6x4 table.
So U=17mm for 15mm games should work on a 180x120cm table with deployment 20U from the centre line.
The proof will be a game.
PS. Making up the ruler was very easy using a Table in Microsoft Word. The settings allow you to specify the cell size in mm. Print, trim, laminate and trim and ruler(s) are ready for use.