You can do that, but it's not too straightforward. Ideally we would have an "input mask" property in the text input control where you can define a format that will be enforced, but that's not the case today - feel free to create a new feature request in the PowerApps Ideas board, or vote up if one already exists.
To implement that today, we would need to update the Default property of the text input on its OnChange property - so that we can check whether it is in the expected format, and fix it if this is not the case. So for the first text input control (which I'm calling "txtReliability", which has the Default property set to the 'readabilityDefault' variable), you can have this expression:
If(
StartsWith(txtReliability.Text, "."),
Set(readabilityDefault, "0" & txtReliability.Text); Reset(txtReliability))
It checks whether the value starts with a '.', and if this is the case, sets the default property with a zero.
The second requirement is a little trickier. We don't have a direct way to calculate the number of decimal places of a number, but we can use some math (and the Log function) to do that:
-RoundUp(Log(Value(txtReliability.Text, "en-US")) + 0.000001, 0)
I used this expression in a label (called 'lblDecimalPlaces', which is visible in the attached app, but would be hidden before you publish it) so that I can refer to that value in the other text input controls.
Finally, we use a similar logic for the text input controls that hold the values for which you want the standard deviation calculated. We use the Text function to format it with the appropriate number of decimal places.
Set(
value1Default,
Text(
Value(txtValue1.Text, "en-US"),
"[$-en-US]0." & Left("00000000000000", Value(lblDecimalPlaces.Text)),
"en-US"));
Reset(txtValue1)
The attached app shows how you could implement this scenario - to open it, download it locally, go to https://create.powerapps.com, select Open, then Browse, and find the file that you saved.
Hope this helps!
