Since you write "Simatic Manager" you probably mean STEP7 v5.x.
In that case, there is nothing exactly the same as the "compute" block in RSLogix.
Closest is to use SCL.
If you want to write the math code in the same block as FBD or LAD code, then you have to resort to STL, which is probably what you want to avoid.
If you consider TIA and the newer PLC types S7-1200 or S7-1500, there is a "calculate" block that works exactly like the "compute" block in RSLogix.