I used the formula LxGxG divided by 400
L= length of pig from between the ears to the base of the tail
G= around the pigs chest behind the shoulders
it helps to distract the pig with treats while your measuring. It also helps to use your wifes flexiable measuring tape. It really helps to get her permission first to use the tape!
this is just an estimated weight but using it we came within 95% of the butchers scale.
You can get a weight measure tape and measure in three places. The first place to measure is around the girth. That is right behind the shoulder. The next place to measure is around the waist. The third place to measure is from behind the ears to the base of the tail. Multiply the three numbers and divide by 400.
That's what I do and it's pretty close.
My friend's have a "pig scale" basically a large durable scale with a narrow metal enclosure and gate at each end. I suppose they're too expensive to get for a single pig.
I generally wait until one steps on my foot and it hurts really really really bad (like I'm cussing outloud bad). Then I know they're probably 300 and it's time to go.