Decreasing in the round for a cone shape

I’ve just about got this whole magic loop knitting thing down, but I’m a little confused with decreasing in the round to get a cone shape. I knit amigurumi dolls and I was asked to make a baby whale. I figured it would be hard to knit it from bottom to top because I’d have to embroider the face.

Once I get ready to decrease, I knit for 3 rounds, but then what do I do? Should I just keep decreasing until I only have 6 stitches left on the needle or do I decrease and knit a couple of rounds and decrease once again until I get my desired shape?

I have 1 more question and I don’t want to fill up this entire board with all my how-to questions so I’ll add it in here also.

Once I finish knitting in the round and I close up my project using the gather bind off, how should I go about making sure that it won’t come undone? I’ve been pulling tightly and then using the mattress stitch to make sure it was secure. I was just wondering if what I’m doing now is correct.

In order to get the cone shape, you may have to experiment with number of decreases and the number of rounds between the decrease rounds, depending on how large the baby whale is. You could try this part out on some scrap yarn, just to get the right slope.
I often finish hats by gathering the remaining sts with a tapestry needle and pulling firmly, then making a small knot through one of the sts on the wrong side and weaving the end through the gathered sts. Thats a very secure finish. If mattress st works better for the shape, weave in the ends and that also should be very secure.
It would be great to see a picture of this finished baby whale!

You will have to experiment some to get the shape you want as Salmonac says, but basically - decreases closer together give you a rounded shaped and further apart gives you cone shape. Think of a smooth rounded cap vs a long stocking cap.

Thank you both very much! I’m going to experiment right now.