The efficiency is actually a property of each reaction alone. But it's hard to calculate from the single one (there are tools to do that, from each amplification curve, but they are not commonly used).
So in theory efficiency should be constant within dilution series and from several known concentration points you can calculate it.
actually, because I got curious about this, I re-assayed my standard, using 1:5 dilution factor. And the result came out quite good, I got slope -3.37. The first 1:5 dilution series, I think I made a pipetting mistake, or other errors.
Now I can use the same dilution factor between the HKG and GOI. Thank you.