[Edit: I was writing this at the same time as Tatewise so apologies if we repeat a lot of similar info !]
Ah OK - I wasn't sure at what stage you were at.
I want to store the entry number of the certificate and also to store the 'Sleeve number' where there is a copy of the certificate. I could use CustomId for one but the other?
In Ancestral Sources there is a reference number text box that can be used for that. Using the default settings this is recorded in the source 'Publication Information' field. The default census title template would record it as part of the source title.
Where to put the district and subdistrict
A problem with addresses in general is it can be very easy for a couple to live in the same house for 40 years but in different records the address can be recorded in various ways. Where possible I would recommend recording the place as consistently as possible across these records and put the more record specific information such as district and sub-district, etc. in the source text. I use the method that most users seem to favour, with the town, county, country (though I don't bother with country as I hardly have any records other that in UK) e.g.
Place: Preston, Lancashire, England
Address: 12 The High Street
Where to put the date of the certificate
If it is a birth certificate then the date of birth would be in the birth event linked to the source. It would also be in the transcription of the certificate in the source text together with the birth registration date.
Also, what do people use the 'entry date' on the citation for?
As all the important data is in the source I don't tend to use the citation to store data. However, it can be used to record the date of the original record. For a birth certificate I suppose this could be the registration date or the birth date. Some people do use it to record the date they entered the new source into Family Historian but I don't think this is what it was intended for.
Hope this helps
Nick