Hi,
When in case of a dual hydraulic failure, slat or flap locked, or whatever it is, and you see "LDG DIST proc.. Apply"
What exactly must be done?
refer to your QRH under Abnormal Procedures, ABN-80 Miscellaneous and there you will find "VAPP calculation". If you follow the flow depicted there you will apply all the appropriate correction factors. In your specific example since the Delta Vref is higher than 20kts you got it right by saying VLS+25, but my suggestion is to refer always to this table, especially in high workload scenarios where your memory could play some nasty tricks. Once you've got the speed, you insert it as you said as VAPP and then if you need to use flaps 3 for landing you select CONF3 from the PERF page menu. You need the landing distance required as well, that you will get from the summary in case of a complex failure (Dual hydraulic, emer elec conf). In the other cases you will need to get the correction factor from the LDG CONF/APPR SPD/LDG DIST TABLE according to your failure and then multiply it by your actual landing distance without autobrake CONF FULL (always regardless of your flaps lever position for landing).
Cheers.