I just took a look at some lines on 5 Ho's and looks to me like ((1xRL)+(2xML))/2 gets ya pretty close. Push comes to shove, the rule of thumb is bet equal amounts on RL and ML to get the equivalent of -1 if I'm not mistaken.
~T~
RL and ML equal payout, resulting number would be the payout at -1.
This one is pretty simple. Not sure where I found it. Hope it helps.
http://zcodesystem.com/runline_calc/