Or just use the table from the rulebook, which gives the direction from a regular d10 roll, based on the direction you tried to throw the grenade.
Sorry, I'm so used to minis it didn't even occur to me. But there's a Scatter Diagram on page 196 of the DH1e Core.
It also has different rules for grenade fumbles, on page 137:
Whenever a jam results from throwing a grenade or firing a grenade, something unfortunate has happened. Roll on the table below to find out the results:
1–5 Dud. The explosive or round fails to explode and, in the case of grenade launchers, the weapon must be reloaded before it can fire.
6–8 “It might be ok…” Nothing happens. Roll again on this table next round.
9–0 BOOM! The round or explosive detonates immediately. Centre the effect on the character. If this was the result of firing a grenade launcher, the grenade detonates in the barrel, having its normal effect as well as destroying the weapon.