It's reasonably standard for an early convertible note to have a discount OR warrant coverage. Anything that gives a permanent fixed percentage throughout future financing rounds will cause you grief. (In other words, the number of warrants are adjusted so that post financing the warrants maintain the same percent ownership as pre financing.)
A professional priced equity round for preferred stock will include restrictions on issuing new stock and will have board and shareholder oversight. Prior to this round, the early warrant holder can be at risk of founders (who control the company) simply issuing new shares and diluting the warrant holders ownership. I have seen where these very early warrants were tied to the ownership of the founders. This would have been okay IF the clause was written to expire when a Qualified Financing occurred. (A Qualified Financing generally is defined as a priced equity round with a raise of X million dollars.)
If this clause does not expire, then it can be a big mess. As a Seed/Series A investor, I have spent many unnecessary hours getting rid of percentage agreements that would have persisted after my investment. It creates an extra burden to the financing. The permanent percent ownership position often is the result of inexperienced investors then who don't understand that they need to accommodate the incoming much larger equity round, or they will kill the company. It is a huge pain in the #$%&@#!
Bottom line: keep it simple. Either a discount or a fixed number of warrants is best. If you must grant a percentage, make certain that it sunsets (expires) with a preferred equity round.