Round up Formula help

Hi, I have been using this formula for cost a calculation but there is an error i would like to correct.

=IF(MOD(100,C13)>1,ROUNDUP(C13*2,-1)-1,(C13-1))

Error:  if the original value is 50 the answer i get is 49 not 99.

The formula should double a numeric value, then round it up to nearest whole multiple of 10, then minus 1 to get the answer.

Example 1: A value of 73.68 would equate to an answer of 149.00.

Example 2: A value of 53.17 would equate to an answer of 109.00.

Example 3: A value of 50.00 would equate to an answer of 99.00.

Example 4: A value of 263.12 would equate to an answer of 529.00.