Hi Elonneke, welcome to the forums 
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The Eastern Europe maps have the same Croatia coverage as the big Europe map. There's no map with better coverage. To check the coverage, you could try this website: http://routes.tomtom.com. You can then decide whether the map fits your needs.
Now that is going to be one of those holiday journeys of a lifetime. I can vouch that the scenery is amazing and you'd better have so much photo storage you'll think it's just too much when you leave Their are towns with modern facilities too so "the old days" are no more.
As to the "60%" and looking at v885 (I hope by now) of the map of the Country some cities and towns seem complete to me so there has been some filling in of the network already. Content ops try to be conservative in their estimates so read 60% as meaning main routes (Highways and feeders and some of the important towns almost and some completed. I've just taken a sample of towns (which you will probably like to explore in or stay over) and I think you'll find the mapping not too shabby, The smaller streets and lanes might not be mapped yet while some towns will just have main routes through town
I'm not familiar with the map version you have right now (can find it using http://tomtom.com/12214)
Haven't yet found out how the chaps & gals specifically judge their coverage figures (though I have an idea).and I would say 60% and growing so v890 (the latest may show more data - certainly road network. The routes will take you via the mostly Motorways which look very good and there are many side roads via interchanges that you will find and these lead to towns. It's the tinier Villages that may not be in the map yet.
