If xbiz or xbder is empty, bc will complain (bash here)

$ echo "5 + " | bc -l
(standard_in) 2: syntax error

You're going to need something like (untested)

if (! $?xbiz) set xbiz = 0
set varx = `echo " 5 + $xbiz" | bc -l`
if (! $?xbder) set xbder = 0
set vary = `echo " 5 + $xbder" | bc -l`

If xbiz or xbder is empty, bc will complain (bash here)

$ echo "5.2 + " | bc -l
(standard_in) 2: syntax error

You're going to need something like (untested)

if (! $?xbiz) set xbiz = 0
set varx = `echo " 5.2 + $xbiz" | bc -l`
if (! $?xbder) set xbder = 0
set vary = `echo " 5.2 + $xbder" | bc -l`