I dislike the idea of having to keep subtracting, as it can be prone to getting out of synch.
I would use a timer set to 0.1sec timebase, with a preset of 4620.
It is simple math then to give the correct RAW output to the analog output card
RawOutput = ((RawMax-RawMin)/TimerPreset)*(TimerPreset-TimerAccumulator)+RawMin
If you are using any of the SLC's or a Micro 1400, you can place this equation in a CPT instruction, otherwise you will have to daisy-chain the conventional maths instructions.
And if you use a RTO instead of a TON, you can "pause" the ramp at any time. If you keep repeatedly pausing your ramp when it uses a TON, you will lose time every time the timer gets disabled, because it will reset.
EDIT: Whoops, I'd forgotten the SLC doesn't have a 0.1 sec timebase, so the best you'll do is 1 sec timebase with a preset of 462