Hi Nobbyknownowt,
I try to avoid userforms if I can get away with it. If you feel the
same way you could use the following series of application.inputboxes
with the input data type restricted to range.
This way is very user friendly because he/she is confronted with one
question at a time; also, they don't have to type in a cell address,
they can input the position simply by clicking on the appropriate cell.
It's also developer friendly, user forms take longer to develop.
The only disadvantage is that the user has to do a bit more mouse
clicking, so if RSI is a problem read no further ;-)

Public Sub weight_positions()
Dim rgW1 As Range 'weight a's cell position
Dim rgW2 As Range 'weight b's cell position
Dim rgW3 As Range 'weight c's cell position
Dim rgW4 As Range 'weight d's cell position
Dim Wa As Single 'I have used 10 g
Dim Wb As Single 'I have used 15 g
Dim Wc As Single 'I have used 25 g
Dim Wd As Single 'I have used 50 g
Wa = 10: Wb = 15: Wc = 25: Wd = 50
Set rgW1 = Application.InputBox(prompt:= _
"Where should the " & Wa & "g weight be?" _
& Chr(10) & "(Click Cell)", _
Type:=8)
Set rgW2 = Application.InputBox(prompt:= _
"Where should the " & Wb & "g weight be?" _
& Chr(10) & "(Click Cell)", _
Type:=8)
Set rgW3 = Application.InputBox(prompt:= _
"Where should the " & Wc & "g weight be?" _
& Chr(10) & "(Click Cell)", _
Type:=8)
Set rgW4 = Application.InputBox(prompt:= _
"Where should the " & Wd & "g weight be?" _
& Chr(10) & "(Click Cell)", _
Type:=8)
MsgBox "a at " & rgW1.Address(False, False) & Chr(10) _
& "b at " & rgW2.Address(False, False) & Chr(10) _
& "c at " & rgW3.Address(False, False) & Chr(10) _
& "d at " & rgW4.Address(False, False) & Chr(10)
End Sub

The msgbox on the last line is just to show you that the code works by
showing the selected cell addresses. Just replace that with your code.

If you also want the user to input the weight values you could include
Wa = Application.InputBox(prompt:="Weight a grams=?", Type:=1)
Wb = Application.InputBox(prompt:="Weight b grams=?", Type:=1)
Wc = Application.InputBox(prompt:="Weight c grams=?", Type:=1)
Wd = Application.InputBox(prompt:="Weight d grams=?", Type:=1)

either all together so that all the weights are first entered, or
position each one before its corresponding cell range input.

Ken Johnson

PS your spreadsheet sounds interesting. If you could send me a copy
when you've finished it would be appreciated. I teach high school
science so it could be useful.