Just so we're on the same page, you're referring to this push-pull plan and not something else, right?
Don't stress over the weight as much as the reps. Use a weight you can handle for whatever the rep range calls for that day. For example, that plan calls for sets of 3 on the heavy days and sets of 6 or 12 on the light days. So, whatever exercise you're using, use a weight that allows only those reps (with solid technique). I'm not sure why this is confusing.
Then you're doing something wrong. It might sound like a meathead maxim, but it's incredibly hard to legitimately overtrain; it's more likely just under-eating and/or under-sleeping. Simple as that.
Let's even say you're getting as much sleep as you can (by the way, in an average week, how many hours per night do you get?)
Are you having a post-workout shake with protein and carbs after each training session?
What's your age, height, and weight?
How long have you been lifting?
Exactly what did you eat yesterday?
Also, if you're new to the plan, there's a chance that you're just not conditioned to the high frequency (training the same bodypart two or more times per week). That's something that can/should be built up within the first few weeks.
If you can handle 205 for 4x8, do you think there's any benefit to doing only 180 for 4x8 and short-changing yourself those 25 pounds per rep?