Need advice re cardio

Bros
Trying to drop some serious body fat and big ol gut, could some of you more experienced members please give me some advice/tips and answers, if possible.

At the moment Im doing 40-50 minutes of cardio every morning on an empty stomach, 4 or 5 times a week. 2 nights a week I will go for a 4 or 5km walk with the wife.

I alternate between the bike and treadmill. Generally the workout looks like this

5 mins - steady light pace
15-20 mins - 3 or 4 sets of high intensity interval, increasing the intensity 1 notch per minute (ie: starting at say 3% incline on treadmill up to 6% and then peaking at 7% as my highest point.
5 mins - lighter pace

I do virtually the same workout but alternate the machines.

I know to lose fat I have to work towards a percentage of my optimum heart rate, whats the best way to calculate this.

Im also taking ECA on my workout days.

Any advice appreciated. Thanks Bros.
DB

Yes you need to keep your heart rate around 65-75% of your age predicted max. This is the formula to use. 220-age=MHR, MHRx65%=??, MHRx75%=??, This will give you a range to keep your heart rate.