You’re using chemicals, so the most important thing is to use only what you need. This means choosing both the right product and the right means of applying it.
In your local garden centre you will find the 4-in-1 products – these contain only a small amount of herbicide and will be of little use. Spreading this over the entire lawn for what may be a few dispersed weeds is simply not good for the environment. A much better option is the spot-sprayer type of herbicide product (and you have much better control over the quantity you use too). Using a watering can to apply your herbicide is also NOT a good idea as a lot of the liquid immediately washes off the weed’s leaves and so you end up applying far too much.
Be patient with herbicides! Doubling the amount you use will not double its efficiency – but it may kill the grass. And even if spot-spraying, treat the weed just once and then wait for up to 3 or 4 weeks before deciding whether a second application is necessary.
If you have a serious weed problem, consider using a reputable and trained lawn care company. They will probably do an initial all-over spray, but it will be done efficiently and correctly.