The angular houses are houses 1, 4, 7, and 10. These houses represent the basic needs, the foundations of life. The cusps of these houses are the angles of your chart– The Ascendant and Midheaven. And their counterparts, the Descendant and Nadir.
The angular houses represent the most fundamental issues in life. Planets in these houses have extra strength and power in the chart.
Your self, appearance, mannerism, approach
Your partnerships (marriage, business partnerships, or any cooperative venture)
Your career, what you offer the world, the role you play in the public sphere
Your home, family, emotions, roots
The cusps of the angular houses are called the angles of the horoscope. Planets near these cusps are most powerful and influential in the person’s life.
Sign on the Ascendant (cusp of 1st)
Describes people’s first impression of you, your personality. Your identity. Your physical characteristics.
Sign on the Descendant (cusp of 7th)
Describes your partner, what you look for in a partner. Aspects of the self that influence attraction.
Sign on the I.C. (cusp of 4th)
Describes your early emotional environment, family of origin. How you like your home to be. Emotional safety.
Sign on the M.C. (cusp of 10th)
Describes your career, achievements and ambitions, what the world remembers you for.
There are many points of symmetry in astrology.
The first house, representing you, is directly opposite the seventh house, representing your partner. The tenth house, representing public life, is directly opposite the fourth house, private life.
You can picture these pairs like two ends of a see-saw calling for balance and co-operation between the two. Balance in relationships (1/7), and work-life balance (4/10).
Any planet situated within 10 degrees of these house cusps, on either side, has a strong influence on that angle and those ideas it symbolizes.
The angles represent the horizon line (the places due east and west of you at the time of your birth) and the meridian line (due south— the highest point on the ecliptic, and due north). These four house cusps will be the same no matter which house system you use. However, for the rest of the house cusps, there are many ideas about how best to place them. Different house systems work well for different astrologers.
My take-away is this: while angular house cusps do have precise locations, the rest of the house cusps seem to be “fuzzy” or less defined. More of a gradient than a well-defined boundary. Therefore, I think you can assume some “wiggle room” with the placement of all house cusps except for the angles.