If you have itxtVal13 = 100 and put a breakpoint on the line
RangeValidatorLYFTE4050.MinimumValue = iminVal13.ToString();
then examine the value of
imaxVal13
what do you see?
You have calculated 250% of itxtVal13 not 150%
Either change the calculation to
int imaxVal13 = itxtVal13 + (itxtVal13 * 50 / 100);
OR to
int imaxVal13 =(itxtVal13 * 150 / 100);
See the difference?
[Edit]
The breakpoint on the line mentioned was just so that imaxVal13 had already been calculated. Your corrected code (this issue only) will look like:
int itxtVal13 = Convert.ToInt32(TextBox1.Text);
int iminVal13 = (itxtVal13 * 0);
int imaxVal13 = itxtVal13 * (150 / 100);
RangeValidatorLYFTE4050.MinimumValue = iminVal13.ToString();
RangeValidatorLYFTE4050.MaximumValue = imaxVal13.ToString();