How do I modify a pattern with a motif in it?

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Question:

I have a pattern for making a heart which is a 21-stitch pattern. I have made a small swatch following the instructions for the 21 stitch pattern, and the heart came out fine. I now want to make a baby blanket with the "heart" pattern incorporated into the blanket. I cast on 100 stitches for the baby blanket (maybe a bit too few stitches, but I'd prefer not to have to start all over as I've already knit 4-5 rows). How can I incorporate the "hearts" into the pattern in such a way that the hearts will be evenly spaced?

By the way, I tried knitting the first couple rows of the 21 stitch pattern, and I did not have enough stitches left to finish the "heart" at the end of the row when I just keep repeating the 21 stitch pattern throughout the 100 stitches. I can figure out that I need to leave some "intermediate" stitches in between trying to start the heart pattern over, what I'm having trouble figuring out is how many "intermediate" stitches should there be before I start over with the heart pattern. If it helps, all the "odd" rows are K2, P17, K2.

I do not have a photo to post. However, if it will help, I can tell you what the instructions are for the "heart" pattern. The first two rows are both Knit, Row 3 (and all "odd" rows) are K2, P17, K2. Row 4 is knit. Row 5 same as Row 3. Row 6 is where the heart pattern begins to take shape. Row 6 instructions are K10, yo, ssk, K9. Row 7 is again K2, P17, K2. Row 8 is K8, K2tog, yo,K1,yo,ssk,k8. Row 9 same as all the "odd" rows. Row 10 instructions K7, K2tog, yo,K3,yo,ssk,k7. Row 11 "odd" row again. Basically the continuing rows start out with decreasing the k by one stitch at beginning of rows.

I hope you understand my question. Thanks for your help.