First of all, joining a curation trail for curation rewards (usually) doesn't work, because the trail votes after the main curator. Curation trails mainly support the authors voted (via author rewards) and the account being trailed (because your vote increases their curation rewards). There may be exceptions, but generally speaking you should only join a curation trail to support the curator and the authors voted, not with the hope of earning big curation rewards.
Your question is still valid, though: In order to provide author rewards to the author and possibly still receive a small fraction of curation rewards from trail votes, your vote has to be "high" enough in terms of SP and vote % as you already mentioned. The main point is, that votes with an equivalent of around 1.2 SP (=a 1% vote at 120 SP, a 10% vote at 12 SP, etc...) are worthless. They don't increase author rewards and are not eligible for curation rewards. Any higher-valued vote is reduced by around 1.2 SP (=a 100% vote with 15SP is "worth" only 13.8SP, a 100% vote at 100SP is "worth" 98.8SP). The smaller your vote, the higher the impact of this 1.2SP reduction. Now it depends on the typical vote percentages of the account you want to trail. If this account is typically voting with 1%, you'd better have at least a few hundred SP to follow with the same percentage. You'd need more if you want to trail with a scaled percentage. As a very rough rule of thumb, use the typical vote percentage of the account you want to trail, scale it with your desired scaling factor and multiply this with your SP - if the resulting value is a single digit number, it's likely not worth it for anybody...