Added 'C' option (Center) to Length. Now length is made from two parts: Mode +/- Value (px/percent)

where Mode can be:
{Empty} - 0
C - half of size
@ - whole size

and Value can be:
00  - pixels
00% - percents

Sample uses:
C+50 - Center plus 50 pixels
C-25% - Center minus 25% of size.
@-9 - Size minus 9 pixels.
29 - 29 pixels.