Here's how I would do it (so it's always /100)
Func _PROGRESS_($a)
Local $Progress_GUI = GUICreate('CONNECTING...' & $a, 300, 100, -1, -1, -2138570616)
GUISetBkColor(0x0, $Progress_GUI)
Local $Progress = GUICtrlCreateProgress( 0, 0, 300, 50)
; GUICtrlSetLimit( $Progress, $a) ; doesn't seem to work here
Local $Connecting = GUICtrlCreateLabel('CONNECTING...', 0, 70, 300, 30, $SS_CENTER)
GUICtrlSetFont(-1, 10, '', '', 'Fixedsys')
GUICtrlSetColor(-1, 0xFFFFA2)
WinSetOnTop($Progress_GUI, '', 1)
GUISetState()
Local $Step = 100/$a, $data
For $i = 0 To $a Step $Step
GUICtrlSetData ($Progress , $i)
Sleep (50)
$data = Int($i * $Step)
GUICtrlSetData ($Connecting, 'CONNECTING...' & $data & '/' & 100)
Next
GUIDelete($Progress_GUI)
EndFunc