Look for (vlax-curve-getclosestpointtoprojection) function... You make line horizontaly like in your picture, then measure - 1.0, then (foreach pt pts => (setq ptn (vlax-curve-getclosestpointtoprojection spline pt '(0.0 1.0 0.0)))... So ptn are your new points on spline - then just use (entmake) or POINT command to place them on spline...