Sage words of wisdom from a guy in the same area as you that wishes he'd found this site BEFORE he built his pond....

1 - DO go to the NRCS office and get their recommendations. And specific to the whistle/culvert size, I recommend something between 150% and 200% bigger than what they say. I used their recommended '100 year rain' sizing and almost lost my pond 5-6 times in a 4 year period before I paid to have the culvert dug BACK out and replaced with one 50% bigger.

2 - PUT IN A BOTTOM DRAIN/SIPHON - ideally with a valve and/or a hookup that could be used - this is my #1 regret. It's a bit pricey to put in but it's IMPOSSIBLE to add later without some serious work/draining your pond/etc. A bottom drain gives you a way to adjust your water level easily, getting rid of the water that's essentially no use to your fish anyway. And having a way to quickly draw down your pond is something I've wished I could do dozens of times since I built the pond. It also might decrease your insurance rates if you have a house or buildings close to the pond as your local fire dept could use it as a water source - depending on how you build it.

3 - READ READ READ. There's a ton of knowledge here. I bet I picked up 15-20 REALLY good tips in my first few weeks on here that I'd have paid good money to have prior to building my pond. Stocking advice, structure construction, etc all is a TON easier to plan for now vs after the pond is full.

4 - if your pond area is anything like mine you probably have ag runoff coming in. If that's the case plan for a riparian area/grassy entry/spot to catch runoff/sediment pond/etc. I sort of lucked into this one, I didn't really plan for that but 2 of the 3 spots I get significant runoff into I was lucky enough to have very shallow, so now I have vegetation there strategically placed to capture as much of the silt (and corresponding phosphorus) as I can. You'll thank your lucky stars on this one when you have less algae than anyone else.

Learn from our mistakes - I've made a ton. And as far as getting a pond builder's help depending on where you are in Central IL I might be able to steer you to some local words of wisdom.