; negative
$before = 0.00002001 * 2 / 2
$after = __num64tostr($before)
MsgBox(0, "before: " & $before, "after: " & $after)

;~ ; positive
$before = 59584773894018382 * 2 / 2
$after = __num64tostr($before)
MsgBox(0, "before: " & $before, "after: " & $after)


Func __num64tostr($dec)
    If StringRegExp($dec, "e") = False Then Return $dec

    Local $split = StringSplit($dec, "e", 1)

    If StringLeft($split[2], 1) = "-" Then

        $split[2] = Number(StringTrimLeft($split[2], 1))
        Local $ret = "0."
        For $i = 1 To $split[2]
            $ret &= "0"
        Next
        Return $ret & Number($split[1]) * (10^$split[2])
    Else

        Local $struct = DllStructCreate("int64;")
        DllStructSetData($struct, 1, Number($split[1]) * (10^Number($split[2])))
        Return DllStructGetData($struct, 1)
    EndIf
EndFunc