Doesn't the sack have suggested distribution settings for both drop and whirlybird spreaders? I don't know the fertilizer brand but some have pictures of what the distribution pattern should look like. Does yours?
For feeding turf we who maintained it aimed to apply 1 pound of nitrogen per 1,000sf per month or extended feeding if it is timed release (weed n feed fertilizers tend not to be long term acting). You said your NPK ratio 18-0-9? Nitrogen content is always represented by that first number. So if you have a 25 pound sack, with 18% of it nitrogen, you have 4.5 pounds of nitrogen, or enough to feed 4,500sf. You just have to adjust for your lawn size to know how much of the sack to apply at one time. Make sense?
Now then, the only catch is you have an herbicide in your mix. This usually does not matter since all typically hinges on nitrogen but look for any package cautions. It also strikes me you are a little late in the season for most broadleaf weed n feed products and you may have missed the crabgrass pre-emergent window as well. Just be mindful of the heat.
I do not know your spreader type. I really like whirlybird type spreaders if yours is not because it is very hard to leave "lap" marks with them. Even the handheld ones work well for modest residential situations.
One think you might do is apply the product at half the rate you determine, once in two directions to get the full amount---especially if your spreader is a drop type. This will also help prevent lap marks.
Hope this helps. If you are not sure, apply conservatively.