Public Variable between Sub and Userform

Hello,

I need to pass a variable and its value from the Sub where it is declared to a Userform.
How can I do it? Here a simple example.

Sub

Public ISplit
Sub Split_Riga()
Dim ISplit As Double
ISplit = Cells(RSplit, 3)
UserForm1.Show
End Sub

Userform

Private Sub CommandButton1_Click()
UserForm1.Hide
a = ISplit
End Sub

Thanks in advance
Regards