Q: About what percentage of circulating platelets are estimated to be held in reserve in the spleen? 

Answer:  About 25%

This explains thrombocytopenia with splenomegaly and thrombocytosis post splenectomy. Venous thrombosis may occur postsplenectomy with platelet counts more than 600 to 800 K/μL. Arterial thrombosis is less likely to occur resulting in CVA or acute MI etc.