Any floor above floor 40 and abive floor 100s+. Also, if you have all sets mythstones or nadroji jewelery, use them to free up tons of set affix space so you can create many possibilities along with 1 aftermath set affix at 5 fkr eg with trickster and others. In 2.1, this will be even better because of the aftermath bonus affix and many other additions. Especially the ed% affix is appearing on a wizard hat with epiphany called epiphany apex and even the ring has the red ed%. You could easily get red ed% from defiant weapons, eternalized /crystalline weapons as well as insolance and nadroji robe along with the new ed%.
Also consider that taunt, fear, frozen, stun and blind immune are all available on ruby. A great build can have its mythic talents such as vanish, bombard, ruptured and crystal affixes which are very important.
Before you try to get major thing like nadrojis robe and insolance and lot of rubys, you must build a farming build.
Start off with a basic farming build with item drops at 200 %, gold find at 650% and luck at 650% and if you have level 99, get fortunate after dealer (provided u can exp farm fast enough for each ascending perk) . If you have hirling, you can get a hidden 100% item drops which goes beyond cap from the 200% maximum hirling and have a look at this gude :
After you have looked at this guide, proceed to make your cheap farming build.
If you get enough resources, have a look at this guide for making sure you have all farming affixes in 1 character while the hirling goes full dps or the other way around.
While you do this, you should consider looking around in the forums for certain builds to understand how they work and how you could add these ideas to make your very own build, possibly ultimate.
Do not 100% copy every build that u want because everything may change and you can never be able to get any build to full potential,not to mention some builds are outdatded. The concepts you can add depending on your purpose though. For fortune bringer, its alright if you copy but there are better options than copying. If you follow all instructions i shown you, you will have spent your crystals properly and its well spent. Before you spend crystals on a full dps build, think, think, think until your 100% sure that it will be ultimate. That means develop your ideas on paper and with your knowledge, try to come up with the ultimate formula and when its finialised, build your build and you will be surpirsed to know how many crystals and mythstones you have.
You can also use Microsoft Excel or some spreadsheet app to organise affixes on your build as well as your own memory. Only do this after you have immediately made your farming build and when you know so much knowledge.
Keep all of what I said in mind and I am encouraging you to always build original build, never copy! That said, it doesn't matter too much if you copy fortune bringer or make a fortune bringer which has exact same affixes but for different classes.