NavList:

Frank wrote

in the UK, and it's equivalent to Pub.249 "Tables for Air Navigation" ("air navigation" in name only) in the US. In those tables, as in Pub.229, one requires an integer value for LHA, but this is not a property of all manual sight reduction calculations. Many good paper sight reduction methods allow non-integer values for LHA and Latitude, and in that case the AP position can be chosen to be identical to any convenient position, like a DR or a destination point (if not too far off).

The obvious example is Ageton HO 211 which uses the DR position and claims not to require any interpolation. HO 200 Aquino is more interesting. You have a choice of  (a) assumed position with interpolation, (b) assumed position without interpolation. The assumed longitude is calculated as part of the procedure, (c) Use DR position. Described as the least desirable for the practical navigator. It involves interpolation and the correction of the results obtained.

I believe that the DR position can be used with 249 and 229 provided that you are prepared to use interpolation but I have not checked.

If the purpose of the app is to check manual working then it should show how the assumed position is derived.  I  get confused trying to decide what value to use to adjust the lat or long and whether to add or subtract it.