I just measured up a spoke and nipple. Both Dt, 14g spoke, 12mm nipple. The spoke can protrude 1.25mm past the nipple before it bottoms on the threads. The groove in the the nipple measures 1.3mm. The threads in the nipple start 3.6mm from the lip.
In theory a calculator tries to give you full engagement, that is to the bottom of the groove in the nipple. So, in theory, you have 2.55mm before you bottom the spoke in the nipple. I still say go 294, if you used the Dt calculator. If it bottoms out I will paypal you a dollar.
