I use a measuring tape and some toy blocks [KEVA planks] on the tablecloth. Sometimes I also have a whiteboard and whiteboard markers too. Better than a gridded battlemat in my opinion.
It doesn't have the issue of diagonal movement and ranging, and it's very easy, so I'd always recommend it. Blocks are optional if they're too much more expensive than a cloth mat.
1" = 5' for D&D, or 1' = 1m for Dark Heresy. [technically, I think 1" ~ 2m on the figures, but the bases are 1" diameter so it's easy to think about]