The hosptial will help you work it out.
As a general guide as a starting point, insulin requirements are reduced by a third.
I presume that you match bolus to carbohydrate intake already?
By way of example, when I started on my pump, my basal insulin requirements were reduced by a third but my bolus ratios remained the same. I have in fact had to incraes my basal insulin quote a bit to get it right - for me thr "reduce it by a third" guesstimate did not work!