Hmm, just read the beginning of this thread again. You're after 1220mm travel on the X-axis but we're discussing buying 1800mm ballscrews, so did you suddenly decide to increase the travel?

300mm is a good width for the gantry, so to get 1220mm travel you could easily use 1550mm(ish) ballscrews. If you're slightly more imaginative with positioning the bearing mounts then you only need the travel plus the length of the ballnut and end-machining, which for RM1610 is 57mm for the ballnut and 76mm for the end machining (including 10mm extra for the pulley). That makes the required length 1220+57+76=1353mm. Call it 1400mm just to 'be safe' and suddenly the motor and ballscrew choice becomes a lot easier as the critical speed is much greater...

On my machine the rails are 2000mm with 2094mm ballscrews and 1700mm travel. The new rails I got are 2200mm, but I'm keeping the travel the same and using the extra 200mm to increase the bearing spacing (among other things!) and using the same ballscrews. This is a similar situation since the frame and rails will be a fair bit longer than the ballscrews.

If you're after more travel then you could get about 1650mm travel with the 1800mm ballscrews.