Dai,
I would work this out as how many years I would expect it to be in service. For example if it were to last for 3 years, I'd divide the total sum by 3 and put a third through over 3 years.
If it was the SLX, I'd probably just put that through as lasting for one year, but I'm not sure how long a SL2 would last.
I'm not saying this is the correct way to do it, but that's the way I would do it.