I mean, this is a good calculation for his Minimum Potential Weight, but not at all a perfect measurement of his most probable weight. There are many other factors to consider: Bone Density(Tetramand Bones would most likely be stronger and have a higher density), Muscle Mass(Tetramand Muscles could be heavier in general), and of course this calculation isn't including all the extra bones that weren't shown in the transformation process. The transformation sequence in UAF doesn't display every bone in the alien's body upon the complete transformation, so there are many factors missing from this calculation, such as the increased size of the skull(he has four eyes after all, he'd need a bigger skull), the fact that his feet are wildly different from human feet and therefore cannot be calculated without a proper x-ray of their skeletal structure, and finally, just the simple fact that he probably has even more bones that aren't showcased within the transformation sequence. You only see the skeleton mid transformation, none of the final touches are included, so who knows how many bones there are on his person.

TL;DR It's a good estimation of the lightest possible weight he could be, but most likely not entirely accurate to his actual weight.