I have not seen a published formula for calculating the package weight that can be secured by molten solder. I have seen forumlas printed for tombstoning. From that aspect, we developed the following rule-of-thumb for first-order estimates:
m = [0.87 gamma(LF) P N]/980
where m = mass (grams) 0.87 is a "safety factor" that assumes a maximum contact angle (theta) of 30 deg so that 0.87 = cos (30 deg). gamma(LF) is the solder surface tension WITH THE FLUX; for Sn-Pb, thats about 380 dynes/cm with an RMA flux P is the wetted perimeter of the I/O (cm) N is the number of wetted I/Os. 980 is acceleration due to gravity (980 cm/s^2).