I have bad news; your idea will not work as stated. When a SCR turns on, the current has to return to below the holding current so it can communicate off. Since the only time this will happen you will have a two speed device, on 100% until the power is turned off, then when the power is on and the SCR is gated it will turn on. If you must use an SCR you will have to force commentate it, this is on the difficult side.
My suggestion is to use a N FET and PWM it. FET, heat sink sizing etc is load and voltage dependent.