We always try to be 'paperless' whenever possible so when it comes to Multi's and series with Bonus caches, we try to do it electronicaly.
When we cached with a PDA as our backup MultiCalc was the business but now we have changed over to using an Android phone as our backup to the Oregon that does not work.

Have now downloaded an application called Geocaching Buddy, with this app you download the cache details onto the device and then you can enter the formulas for each additional waypoint and then enter the digits as an when you find them. When all of the digits have been entered the waypoint co-ords are displayed and then you can either manually enter them into your GPS unit or use the built in 'Navigate' function which will display a basic compass, distance and destination and current co-ords.
Have tested it out on 2 caches and it was spot on both times.
It takes a little getting used to at first but once mastered is easy to use.
Especially handy for those Multi's with three or four locations to calculate and visit, due to the fact that it can add multiple waypoints and formulas.
Very handy when you do a large series in segments as there is no risk of loosing the bits of paper you note the Bonus numbers on.
It is not free but at 7.99 euros it is good value.

Search for Geocaching Buddy in the App store, also available for the iPhone.