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On new properties IVA has to be paid - there was a special reduction down to 4% but I think that has ended now and I think the current rate is 8%. On secondhand properties a transfer tax is paid and that can vary by region but is currently 6-7%.
The old 10% rule is therefore a bit out of date and I think safer to allow say 12% but take professional advice as tax rates are constantly on the move at the moment.

Thanks for the info. What additional costs make the 5% differential between "Tax" and the total like, likely % cost of Abogado, Gestoria and is there likely to be anything else?
With regard to your remarks concerning fluidity of tax rates, is the current trend up or down in your opinion ?

Actually I have been making enquiries this morning and discovered that IVA is now 10% and transfer tax 8% (might vary by region)
It depends on which method you buy but yes notary, solicitor, land registry all incur costs.
So if it is new allow 14% to be on the safe side. Better to have money over than be short.
Obviously a mortgage will also incur set up costs.
Some of these costs will also vary depending on being resident or not and whether it is your primary residence or a holiday home.
As I said take professional advice from a good solicitor who is totally independent of the sellers.

What would be a good rule of thumb for annual property tax assessments?

Its right. 10 %

When we bought our house 11 years ago it was about 10 % over the price of the house
that included all fees , tax etc .

10% are tax. IVA if new built. Transmisiones Patrimoniales if it´s second hand.

Between 1,5 and 2% for notary, land register, solicitors...