A quick rule of thumb to conservatively calculate the max lift-off weight in pounds is to take the average thrust in Newtons and divide by 25 for ~5:1 thrust to weight ratio. (Actually 5.6:1 since the actual divisor should be 5 * 4.45 = 22.25 but 25 is easier to do in your head.)
On a windy day, you should be using a 10:1 ratio so you should divide by 50.
Clusters are trickier. For a cluster, to be conservative, it is best to be able to lift-off safely if only 1 engine lights, so for a 2 engine cluster of the same engine, divide the total average thrust by 50, 3 engines divide by, 75, n engines divide by n * 25. For large clusters this may be too conservative, so you may want to divide by (n-m) *25 where m is the probable number of misfires.
So for a dual cluster of G35's, I would recommend (35+35)/50 = 1.4 pounds.
If the clusters are made up of a combination of engines, it's even trickier. Smaller engines light quicker, so it is possible that the smaller engines will light before a bigger main engine. An example would be G or H engine clusters around a J. The smaller engines will light first, and the bigger engine might not light before the rocket lifts off. If it lights in the air, the rocket accelerates non-vertically the air. I've seen this happen and it's scary.
Each case is different, but a suggestion is to choose a main engine that is capable of lifting the rocket safely by itself, and to choose the smaller motors so that they won't launch the rocket without the main. Another solution is to have on-board pryo circuitry to light all the motors. It's very conservative, but it's safe.
Bob Krech