Activity Split Relationship

you can do that or 

have B be a start to start successor to A with a lag equal to 20% of the time this is the same as what you are proposing by splitting the activity. The problem with your method and this method is that 20% of the time may not equal 20% progress. 

Thanks for your suggestion Zoltan.

P6 does not provide for %volume Lag but only Time Lag.

P6 does not provide for adjusting Time Lag.

https://www.ronwinterconsulting.com/Making_CPM_Transparent.pdf

In-Progress Lag Report and Value: Remaining Lag should be displayed just as remaining duration is shown and editable. The CPM feature of Remaining Duration was added so that Schedulers could monitor and change this calculated duration result. Lags are to relationships what durations are to activities. Why should Remaining Lag calculations be discarded by the software instead of saved and displayed? Why should we be unable to indicate that actual remaining lag is only 2 days instead of the calculated 4 days? Part of the reason CPM Schedules are so hard to understand is that fact that not all of the data used is displayed.

A workaround is to use activities to represent lag.  This will make lag more visible, will allow for lag calendar be independent of successor/predecessor, and will allow adjusting remaining lag.  It will also allow to model lag durations in Monte Carlo simulations when your software does not provide for probabilistic Lag durations. To avoid confusion be consistent, if using activities to represent lag then model all lag as an activity. You can use an activity code to identify lag activities and to filter them.

 click image to zoom

Good Luck,

Rafael