I use Excel to record my investments. I project numbers on the basis of "If
the rate of change so far in this share continues for the rest of the year,
what will be the year-end outcome?" My cell formulae work well, but . . .
(a) How do you restrict the answer cell from being worse than -100% (I
can't lose more than my original investment), and
(b) How do you prevent the £ or $ amount being less than zero?

For the life of me, I cannot find the answer in Excel Help. I have tried
using 'Solver' and 'Data Validation', but having entered the values I want,
my instructions are then ignored.