Perception
Source GM Core pg. 118
Perception is a fairly straightforward statistic. Use Wisdom as a guide for setting it, and adjust to the high side if your creature has acute senses or extra training. If your creature has low Wisdom, for example, it would probably have a low Perception modifier, or moderate if it's supposed to be a great hunter. Don't make your creature's Perception higher just because it's often used for initiative; creatures with poor Perception could use a skill check for initiative instead, such as Stealth.
Senses
Source GM Core pg. 119
Choose or design any special senses for your creature, such as low-light vision, darkvision, or scent. If you're making a sense from scratch, simply decide what it senses, whether it has a range limit, and whether it's precise or imprecise. For example, a ferrofluid ooze has magnetic scent. This means it can sense large sources of magnetic metal within 100 feet, and the sense is precise—about as acute as human vision in daylight.
Perception
| Level | Extreme | High | Moderate | Low | Terrible |
|---|---|---|---|---|---|
| –1 | +9 | +8 | +5 | +2 | +0 |
| 0 | +10 | +9 | +6 | +3 | +1 |
| 1 | +11 | +10 | +7 | +4 | +2 |
| 2 | +12 | +11 | +8 | +5 | +3 |
| 3 | +14 | +12 | +9 | +6 | +4 |
| 4 | +15 | +14 | +11 | +8 | +6 |
| 5 | +17 | +15 | +12 | +9 | +7 |
| 6 | +18 | +17 | +14 | +11 | +8 |
| 7 | +20 | +18 | +15 | +12 | +10 |
| 8 | +21 | +19 | +16 | +13 | +11 |
| 9 | +23 | +21 | +18 | +15 | +12 |
| 10 | +24 | +22 | +19 | +16 | +14 |
| 11 | +26 | +24 | +21 | +18 | +15 |
| 12 | +27 | +25 | +22 | +19 | +16 |
| 13 | +29 | +26 | +23 | +20 | +18 |
| 14 | +30 | +28 | +25 | +22 | +19 |
| 15 | +32 | +29 | +26 | +23 | +20 |
| 16 | +33 | +30 | +28 | +25 | +22 |
| 17 | +35 | +32 | +29 | +26 | +23 |
| 18 | +36 | +33 | +30 | +27 | +24 |
| 19 | +38 | +35 | +32 | +29 | +26 |
| 20 | +39 | +36 | +33 | +30 | +27 |
| 21 | +41 | +38 | +35 | +32 | +28 |
| 22 | +43 | +39 | +36 | +33 | +30 |
| 23 | +44 | +40 | +37 | +34 | +31 |
| 24 | +46 | +42 | +38 | +36 | +32 |