I have arranged for a couple of day trips for our group of 12 while we are in Tuscany. We'll have two drivers who will also serve as guides for the day. Can anyone give me a guideline for good service?
We'll also have a cook for a couple of nights , what percentage there?

Not sure why you would tip a tour guide or a cook when in Europe they will be paid by their employers or be self employed.

As far as the tour guides, these work for a company.

Indeed I understand that on group bus tours for example the guides depend totally on tips and at the end of the trip the leader will make sure everyone knows that.

I usually use Context Tours for a 3-4 hour guided walking tour and the groups are always small (6-8 people). The tours usually cost approximately 80 euro per person on average. I usually tip 15-20 euro to my guide if they were knowledgeable and did a good job, which they usually do. If, by chance they were average I will tip 10 euros to the guide.

Recently I had a full day tour of three wineries in Umbria which included three wineries and lunch. It was a private tour with guide who drove me around for 250 euros. The guide was extraordinary and provided buckets of information beyond what I initially requested. I ended up tipping him 60 euros.

5 to 10% tops, this is Europe, I'd probably offer nothing unless the staff had gone above and beyond

I always tip the Tour Guide.

They may get paid by the Tour Company but they rely on tips also.

I have had tour guides that were so good that they virtually made the whole day.