Moving 16 studs in the direction of the origin to 25, 0, 25 is not actually going to give you (8, 0, 8), it would actually give you approximately (11, 0, 11) because the vector is going in a diagonal direction. You can read up on Vectors to understand this

I’m not sure if (8, 0, 8) would be your desired results, but if you actually want to move 16 studs in the direction of (25, 0, 25) from the origin, here’s how you would calculate alpha, or the lerp percentage:

local studs = 16
local mag = (a - b).Magnitude
local lerpAlpha = studs / mag