I usually go by ratio 100 grams DME to 1000 ml of water, ends up in OG 1035 which is a good target.
Its better to do a little overpitching then underpitching, I would go for maybe 1,2 Liter and 120 grams of DME.
If you have fresh yeast you could probably go for a smaller starter, maybe 800-1000...