Transact SQL :: Query To Avoid IF And CASE And Calculations Carried Out Once Only To Speed Up With Common Comparing Columns

Got a query taking too much time because of lack of cross columns MAX/MIN functions. Consider a similar example where a View is required to reflect distribution of Water among different towns each having four different levels of distribution reservoir tanks of different sizes:In this case the basic table has columns like:

PurchaseDate
TownName
QuantityPurchased
Tank1_Size
Tank2_Size
Tank3_Size
Tank4_Size

Now suppose I need a query to distribute QuantityPurchased in the Four additional Columns computed on the basis depending on the sizes declared in the last four fields,in the same order of preference.For example: I have to use IIFs to check: whether the quantity purchased is less than Tank_A if yes then Qty Purchased otherwise Tank_A_Size itself for Tank_A_Filled

then again IIF but this time to check:

Whether the quantity purchased less Tank_A_Filled (Which again needs to be calculated as above) is less than Tank_B if yes then Tank_A_Filled (Which again needs to be calculated as above) otherwise Tank_B_Size itself for Tank_B_Filled