My Jigger is in ml but most of the recipies I work with are in oz. What I tend to do is just use the measures as a ratio guide taking into account my jigger is divided between 25ml and 50ml so:
1 shot = Approx 1.5 oz or 50ml
so if I have a recipe that calls for:
2 oz Whiskey
1 oz Grenadine
0.5 oz lime juice
4 oz cola
Using the large jigger that translates to:
1 & a 1/3 shots whiskey
2/3 of a shot of Grenadine
1/3 of a shot of lime juice
2 & 2/3 shots of cola
as long as the ratios are the same then the drink should taste fine (you might just end up with a slightly smaller or slightly larger one than the exact messures).